The present invention relates to a coating method for coating a flexible support with a coating composition mainly containing an organic solvent, and particularly to a coating method in which a running flexible support laid between pass rolls is coated with a coating composition while a coating head having a back edge and a doctor edge is pressed to the flexible support.
Heretofore, various apparatuses have been proposed as coating apparatuses of the type of coating a running support with a coating composition. As one of this type of coating apparatuses, there is a coating apparatus in which a coating composition mainly containing an organic solvent is applied. The coating apparatus for application of a coating composition mainly containing an organic solvent is used, for example, for cleaning a support or forming an undercoating layer thereof before application of a magnetic dispersion or for forming a back layer in the case of a magnetic tape or the like.
As a method for forming an undercoating layer as described above or the like, there is a method as disclosed in Japanese Patent Unexamined Publication No. Sho-57-156066, in which after a running support is coated with a coating composition of a coating composition tank through a rotating coating drum, the thickness of a coating film is adjusted by a coating film thickness adjusting member to obtain a desired coating layer. Here, a metering means (quantification means) such as wire knife, blade, wire bar, etc. is used as the coating film thickness adjusting means. Further, a rod member which rotates in a direction reverse to the direction of running of the support can be used in the apparatus.
On the other hand, Japanese Patent Unexamined Publication No. Sho-62-60750 has disclosed an apparatus in which a slit is provided to be capable of supplying an organic solvent to a running support and in which a surface of the running support is cleaned by scraping off an organic solvent type coating composition with use of a rod member rotatably provided at a top portion of the slit while rotating the rod member suitably and ejecting the coating composition from the slit.
As described above, various apparatuses using a process of application of an organic solvent type coating composition, for example, as a process before a process of application of a magnetic dispersion have been proposed and employed conventionally.
Of the coating apparatuses for application of an organic solvent type coating composition, the apparatus for application of a coating composition from a slit under suitable pressure (liquid pressure) as disclosed in Japanese Patent Unexamined Publication No. Sho-62-60750 has an advantage in that the apparatus is adapted to high coating speed. That is, air which is associated with the support as the liquid pressure of the coating composition ejected from the slit is increased can be removed smoothly. That is, the apparatus contributes to stabilization of high-speed coating.
In a coating head used in the conventional coating apparatus, there is however a large problem when the liquid pressure of the coating composition is increased. This problem is in that a mist of the solvent of the coating composition is produced in the upstream side in the direction of running of the support (in the upstream side of the slit) because the internal liquid pressure of the coating composition ejected from the slit of the coating head is instantaneously released to atmospheric pressure when the coating composition is ejected from the slit. When a large quantity of the mist is produced, there arises a problem that a solvent atmosphere is increased so that work environment is made undesirable.
In the method of performing application of a magnetic layer after application of a pre-coating composition mainly containing an organic solvent in a process of producing a magnetic recording medium as described in Unexamined Japanese Patent Publication (Kokai) No. Sho. 63-20069, there arises a problem that the mist of the pre- coating composition is deposited onto the magnetic layer again to cause coating film failure.